1. Field of the Invention
The present invention relates generally to compasses, and more particularly, to a compass which utilizes a compass card intermediate a light source and a photon detector means. The compass card has a light transmission slot which varies in width, the light transmission slot attenuates the light intensity mechanically in such a manner where the instantaneous light intensity transmitted therethrough itself gives positional information.
2. Description of the Prior Art
Compasses which utilize a rotating disk with a light attenuation means located thereon have been disclosed in the form of an optical shaft encoder and are well known in the art.
A typical example of such a device is U.S. Pat. No. 4,117,602 issued to Lapeyre. This device incorporates a light emitting diode to illuminate a series of tracks on a coded disk. The tracks located on the coded disk are a plurality of alternating light transmissive and light opaque regions which represent one bit of a multiple bit digital code. A sensor array senses the light transmitted through the disk as the disk is rotated by a magnet means and through appropriate signal processing is able to decode the signals into directional information.
Other means to derive information from a light source and a compass card is shown in U.S. Pat. No. 4,577,414 to Migliori et al. This device includes a compass disk which is rotatable and has a magnet means and a light polarizing means located thereon. A light beam is split into a plurality of beams two of which pass through a first polarizing means and two of which pass through a second polarizing means, the second polarizing means having their optical axes at right angles to the first polarizing means. By comparison of the ratio of the intensities of the beams which pass through the polarizing means one may determine the angle of the magnet means in relation to a fixed axis of the compass body.
U.S. Pat. No. 4,720,631 issued to Lapeyre discloses a compass card which has a linear photodetector array in a spatial relation with a light source. Intermediate the photodetector array and the light source is a rotating disk with slots located thereon. As the disk rotates the light is passed through the slots in such a manner that the linear photodetector array may detect a certain or specific random pattern of illumination which corresponds to a specific disk position. The signals generated by the photodetector array are decoded on the basis of a unique positional calibration stored in a read-only-memory and then output to the user.
Thus, while the foregoing body of prior art indicates it to be well known to use encoder shafts and other means to determine position by utilizing digitizing means and polarization means, the provision of a more simple and cost effective electronic compass device which may be mass produced and utilized by the common man is not contemplated. Nor does the prior art described above teach or suggest a simple compass device which utilizes a compass card having a light transmission slot which varies in width whereby the light transmission slot attenuates the light intensity mechanically in such a manner where the instantaneous light intensity transmitted therethrough itself gives positional information.
The foregoing disadvantages are overcome by the unique compass card of the present invention as will be made apparent from the following description thereof. Simplicity is evident over the prior art in that the response function is a linear function of the source emission intensity striking one or more detectors as described herein. The linearity is determined by the linearly variable width of the slit and the source detector means employed, the source detector being selected to have a linear response to the incident intensity. Further, neither expensive polarizing sheets or fiber optic cables are needed nor are extensive read-only-memory to store positional correlation information. Other advantages of the present invention over the prior art also will be rendered evident.